Pebble heat-exchanger



Patented Apr. 21, 1953 PEBBLE HEAT-EXCHANGER Robert R. Goins, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application May z, 1949, serial No. 90,921

(ci. 19a-55) 11 Claims. l

. This invention relates to improved pebble heatexchanger design and to methods for effecting improved pebble and gas ow through such exchangers.

Pebble heat-exchangers are nding increasing favor in heating gases, such as air, nitrogen, etc., and in eiecting various chemical processes in the vapor phase at elevated temperatures, especially hydrocarbon conversion processes. A convenventional pebble heat-exchanger unit comprises a pair of vertically aligned pebble heatexchangers connected by an axially positioned throat therebetween so as to provide for gravity iiow of a contiguous stream of heat-exchange pebbles through the heat-exchangers. The pebbles in the upper heat-exchanger are contacted with hot combustion gas (or other hot gas) formed either in burners adjacent the base of the upper heat-exchanger or directly in the pebble mass itself. In this manner, the pebbles are heated to any suitable temperature above the required ,heating or reaction temperature in the lower heat-exchanger. The hot pebbles then gravitate to the lower chamber where they are contacted with the feed gas to be heated, treated, and/or reacted. The lower heat-exchanger usually has a funnel-shaped bottom converging to a pebble conduit which leads to the bottom of a pebble elevator or lift, usually of the bucket type, but which may be of the screw or air type. The elevator transfers the pebbles to a pebble chute above the level of the top of the upper heatexchanger and this pebble chute or conduit leads into the" vupper portion of the upper heatexchanger. This arrangement provides for continuously heating one section of a gravitating mass of pebbles and simultaneously continuously heating a gas in the lower section of the gravitating mass of pebbles and returning the cooled pebbles from the lower section of the gravitating mass to the upper section for repeating the cycle.

Pebbles utilized in the process and apparatus with which this invention is concerned are compact, spherical units consisting of alumina, mullite, zirconia, thoria, periclase, magnesia, high temperature alloys, such as Monel and inconel, and in some cases where temperature requirements are not too severe, metal such as iron, nickel, chromia, etc. In some processes, pebbles which will withstand temperatures upwards of 3000" F. are required and several good pebbles which function at these temperatures have been developed, one of the most suitable being mullite-alumina. Pebbles range in size from about l/a to 1" in diameter, but usually are within the range of 1A t it.

One of the chief difficulties of pebble heatexchanger design is in designing the exchanger so as to obtain uniform pebble flow therethrough. Uniform pebble iiow in the pebble heating chamber is important because without it, over-heating of some of the pebbles will occur while some sections of the gravitating pebble mass will be underheated. Conventional design provides a cylindrical chamber with a hopper or funnel-shaped bottom which directs the flow of pebbles into a relatively narrow throat leading to the lower heatexchanger. This corresponds to the general shape of the lower heat-exchanger also. It is found that pebble llow in this type of -vessel is non-uniform through a considerable portion of the vessel. Stagnant flow areas exist chiefly around the juncture of the corneal bottom and the cylindrical sides of the vessel, while extremely fast now areas are found around the axis of the vessel. This non-uniform ilow of contact material in a cylindrical vessel is emphasized in the U. S. Patent 2,430,669, to John A. Crowley. While the patent is not concerned with pebble heatexchanger operation but rather with the iiow of irregular particulate contact material, the flow problems are similar.

Another diiculty encountered in pebble heater operation and design lies in obtaining uniform gas iiow upwardly through all sections of the heatexchange chamber. Without uniform gas flow, it is obvious that there will be a lack of uniform heating, and especially treating or reacting of the gas in the lower chamber. In the conversion of hydrocarbons, particularly, uniform gas low is paramount in order to control the exact contact time required in most conversion processes and especially processes which are effected at the high temperatures provided in pebble heater processes. It has `been found that over-contacting or soaking of the hydrocarbon in the heat-exchanger results in complete cracking to coke and deposition of the same on both the pebbles and the interior of the chamber which has sometimes resulted in shutting down of the unit due to clogging of the pebble passageways with fragments of coke from the walls of the exchanger or agglomeration of the pebbles with carbon.

The principal objective of the present invention is to provide uniform pebble and gas ilow in a system of pebble heat-exchangers. Another object of the invention is to provide uniform contact between gas and pebbles in a pebble heatexchanger. A further object of the invention is to provide uniform gas-pebble contact with a` minimum of pressure drop through the pebble bed. Other objects ofthe invention will become ap- 3 parent from va consideration of the accompanying disclosure.

The present invention provides a pebble heatexchanger design which requires relatively short horizontal gas flow through the pebble bed and therefore permits the gas to traverse the whole horizontal. cross-section of the bed on its way upwardly through the chamber. It also provides for the introduction of the gas to the pebble bed at a point above the area of stagnant flow so as to contact the pebble mass only in that section which is moving uniformly toward the pebble outlets as a preferred modification of the invention.

In order to improve the flow ofv pebbles through the heat-exchanger, the present design provides a relatively narrow pebble column above each pebble outlet. It has been found that in order to obtain any degree of uniformity of pebble flow, even in the upper section of the bed, the height of the bed must be at least one and one-half times the horizontal width of the bed in a vessel where the pebble outlet is positioned atthe center of the bottom. By effectively dividing up the gravitating pebble mass into several narrow columns with a separate pebble outlet for each, the effective height of the column is increased and its width decreased, and it is therefore found possible to utilize the upper section of each column as a uniformly gravitating mass for contact with the contacting gas admitted at points above the bottom of the column. The uniformly gravitating section of the pebble bed in apparatus of the invention includes the upper three-fourths to seven-eighths of the bed. rIhe high narrow columns of flowing pebbles provide maximum uniformity of ow without' cutting down the effective Width of the entire pebble bed and hence, the capacity of the unit. The effective narrowing of the pebble column is combined with the injection of contacting gas at points closer to the center of the column than is provided in conventional pebble heat-exchangers.

In order to more clearly understand the invention, reference is made to the drawing of which Figure 1 is an elevation, partly in section, of a two-chamber pebble heat-exchanger system or unit; Figure 2 is a horizontal cross-section of the heat-exchanger shown in Figure 1 taken on the line 2-2; Figure 3 is a vertical cross-section of a heat-exchanger taken on the line 3-3 of Figure 2; Figures 4 and 5 are horizontal cross-sections of other modifications of heat-exchangers.

Figure 1 shows a pair of pebble heat-exchangers I and II connected by connecting section I2. The unit is lined with a refractory I3 which may be of any suitable high temperature refractory composition, such as electric furnace alumina. Refractory columns I4 and i5 are positioned axially in each chamber so as to provideV ai somewhat annular pebble bed of narrow horizontal cross-section in relation to its height. Element I6 is a pebble chute connecting the top of the elevator andthe upper portion of the interior ofthe upper pebble chamber through stack I'I'. The lower end of pebble chute I6 terminates ina. plurality of short feeder tubes I3 which deliver pebbles to the center of the generally annular pebble bed 2| surrounding column IIL In a similar manner, pebble bed 22 in the lower chamber is supplied. continuously by pebble passageways'or throats 23 leading from the bottom oftheupper chamber tothe top of y positioned similarly in the bottom of the lower chamber and function to pass pebbles continuously into the hopper-shaped section 25 below the floor of the lower chamber. This hoppershaped section converges pebbles to pebble conduit or chute 26 leading to the bottom of elevator 28 through pebble feeder 21. This feeder 21 may be any type of devicev used for feeding small particulate material through a conduit, such as a table feeder, a star wheel, a vibrator screen, etc. Elevator 23 is preferably of the bucket type but may be of the screw or gas lift type.

Conduits 29 and 3| positioned in the lower portion of each heat-exchanger serve to introduce gas in heat-exchange relation with the pebbles in the chambers. 'Ihe exact arrangement of these conduits can best be seen in the other figures of the drawing. A combustible gas introduced through conduits 3I is introduced to chamber i0 where it is burned. The combustion gas passes upwardly through the chamber, heating the pebbles therein, and out through stack I1. In the lower chamber the feed gas to be heated, treated, and/or reacted is introduced through conduits 29 and the effluent is taken off through an opening `36 in the dome of the chamber into a gas space 34 in the'center of connecting section I2, from which it is re'- moved through one or more openings 35 in the Wall of the gas space. For the sake of simplicity, control means, motors, recorders, blowers, etc., have not been shown.

An essential feature of the design of the unit lies in the relatively narrow elongated pebble throats or passageways 23 in Figure 1 which function as valves when full of pebbles to minimize the ow of gas between the upper and lower heat-exchangers. In order for these pebble passageways to obstruct the flow of gas between' chambers, the whole unit from pebble inlet nozzles or tubes I3 to pebble feeder 21 must be maintained full of a contiguous compact bed of pebbles. While pebble passageways 23 and 2li` should be relatively narrow, they should be sufiiciently large in diameter to provide free flowv of pebbles therethrough and in any case should be several pebble diameters across.

Figure 1 shows a plurality of pebble feeder pipes I8. Any convenient number of these may be utilized so as to properly distribute the pebbles around the bed. While this arrangement provides better pebble distribution over the top of the bed than would be provided without the use of these tubes, the apparatus will function satisfactorily if all. of the pebbles are delivered at a central point from the mouth of pebble chute or conduit I6. Likewise, it is not essential to the design or"- the apparatus to position the lower end of conduit E6 axially inside of stacki1. However, this arrangement utilizes the heating effect of the combustion gas to a maximum by heat-exchange with the incoming pebbles which are at their lowest temperature in the system. Because of the lower temperature existing in the upper section and stack I'I of the upper heat-exchanger, elements I 6 and. I8 may be fabbricated of metal, such as stainless steel, or even less expensive metal, which is non-corrosive and will withstand temperatures up to 800 to 1000 F.

rEhe design of Figure 2 is that of a heatexchange chamber dividing the pebble bed into four symmetrical columns grouped around the four pebble outlets 23 and bounded by the cylindrical refractory wall I3, refractory ridges 732, and

the center column I4. These refractory ridges 32 preferably extend upwardly from the floor of the chamber to the dome thereof so as to provide a pebble column of uniform horizontal crosssection. These arcuate ridges curve inwardly so as to narrow the width of the pebble column between each pair of ridges and between each ridge and the center column I4 and to withstand the horizontal thrust of the pebbles against the wall of space 30. Ridges 32 are preferably hollow at their lower extremity as shown in Figure 3 where hollow space 33 is shown as a combustion chamber into which a combustible gas is introduced through line or conduit 3l. Openings 33 in the upper part of the combustion chamber (or gas collecting space when this design is used for the lower heat-exchanger) lead slightly downwardly into the pebble chamber and radially from the arcuate section or ridge so as to properly distribute the gas over the full cross-section of the pebble column. Gas passageways 33 are directed downwardly into the pebble chamber so as to prevent the flow of pebbles into combustion chamber 30. Element 35 is the effluent outlet shown in Figures 1 and 2 for removing gas from the lower pebble heat-exchanger. While a single conduit is shown, any suitable number of these gas take-olf conduits may be used.

' Figure 4 shows a modication of the invention without the center column. This design effectively divides the heat-exchange chamber and pebble bed into three vertical columns bounded by the cylindrical refractory lining of the chamber, the arcuate sections, and an imaginary surface extending from the center of each arcuate section to the axis of the chamber. Pebble outlets 23 are positioned at the approximate center of the bottom of each column so as to withdraw pebbles uniformly therefrom, and improve the pebble now through the entire cross-section of the heat-exchange chamber.

Figure 5 illustrates another modication of the invention without the center column which permits direct introduction of a combustible gas to the pebble bed and burning of the combustible mixture on the surface of the pebbles in the lower portion of the chamber. This unit may also be operated by introducing hot combustion gas directly through line 3|, but this method of operation is not so efficient because the hot combustion gas must be formed outside the chamber and some loss in heat will be involved. Where it is preferred, the heat-exchangers shown in the first four figures may be modified in the same manner as shown in the design of Figure 5 so that the contacting gas is introduced directly from outside the chamber through the inwardly extending ridges or arcuate sections. The combustion or gas space 30 shown in Figures 2, 3, and a is not so essential in the gas treating chamber or lower heat-exchanger although this type of gas collecting space may be utilized in the distribution of the gas to be treated to the pebble bed in the lower heat-exchanger.

A significant feature of the invention and of all of the modifications 'shown is the introduction of the contacting gas in either chamber at a level above the bottom of the chamber, as shown in Figure 3, through holes 33 in the refractory wall of the combustion space. irrespective of the design of the heat-exchange chamber, there will always be a small stagnant flow area in the bottom of the chamber between the pebble passageways and the vertical wall or walls of the chamber. In the heating chambers of the invention 6 the uniform pebble ow section occupies at least the upper three-fourths and up to seven-eighths or more of the pebble bed. By introduction of the gas into contact with the gravitating mass of pebbles above this stagnant area, contacting is effected only in the uniform pebble-flow section of the bed `which results in more nearly uniform contact time between gas and pebbles in various sections of the bed. i

The effective narrowing of the pebble bed provided by the invention is obvious from ay consideration of the designs sho-wn in the various gures of the drawing, and this is effective in reducing the horizontal distance which the contact gas must travel in order to traverse the entire cross-section of the pebble bed. 'Iliis horizontal distance is shortened by application of the invention to a pebble heat-exchanger of any given diameter. i y

Pebble heat-exchangers of the type with which this invention is concerned have utility in a wide variety of heat-exchange processes which require the heating, treating, and/or reaction of a, gaseous feed. The application of M. O. Kilpatrick, Serial No. 761,696, led July 17, 1947, describes the detailed operation of a pebble heat-exchanger unit in the conversion of hydrocarbons. The application of C. W. Perry, Serial No. 677,357, filed June 17, 1946, now U, S. Patent No. 2,596,507, shows the use of pebble heat-exchangers in the synthesis of HCN from NH3 and carbon-contaming gases. The application of Sam P. Robinson, Serial No. 665,673, led April 29, 1946, now U. S. Patent No. 2,551,905, illustrates -the use of pebble heat-exchanger apparatus in the desuliurization of gases, particularly hydrocarbon gases. The use of pebble heat-exchangers in the synthesis of carbon disulfide from hydrocarbon and sulfurcontaining gases is disclosedin the application of Sam P. Robinson, Serial No. 651,293, led March 1, 1946. The heating of air for high temperature use inthe fixation of atmospheric nitrogen in a pebble heat-exchanger is disclosed in the application of Sam P. Robinson, Serial No. 767,300, filed August 7, 1947. The process of this last application illustrates the use of a' heating gas other than combustion gas for heating the pebbles in the upper heat-exchanger. In the process referred to, air heated in the lower heatexchanger is mixed with fuel and burned inra high temperature furnace to produce a temperature of approximately 4000 F. so as to effect the xation of nitrogen and, in order to prevent de- 'composition of the product, the hot effluent is immediately passed through the upper heatexchanger in contact with relatively cold pebbles, thereby heating the pebbles and quenching-"the product effluent from the reaction in the furnace. The hot pebbles gravitate to the lower heatexchanger and are there contacted with atmospheric air so as to pre-heat the air for the fixation process.

The instant invention is applicable to the processes to vwhich the applications just referred to relate. Since the pebble heat-exchanger disclosed provides uniform pebble flow and gas flow, it is particularly applicable to the conversion of hydrocarbons and to chemical processes which require short, specific reaction times. In hydrocarbon conversion processes involving dehydrogenation and cracking to specific products without over-cracking, the present invention offers particular utility.

I claim:

1. A pebble heat-exchanger for effecting heatexchange-between al gravitating bed of pebbles and raf gas,-comprising a refractory-lined upright closed. cylindrical vessel. enclosing a heatexchange.y chamber.. of symmetrical generally circular horizontal cross-section modified by the inward projection of .f several Avertical. ridges uniformly; spaced-onfthe innerwallthereof extending; from: the: bottomJ upwardly, intothe upper section of said chamber so that verticalv radial planespassing thru the rcenter of theridges divide. thechamber intoa plurality of contiguous vertical columns; gas inlets ineach of said ridges in the :lower portion-thereof, the remaining portionoffthe.ridgesfbeing imperforate; a refractory-floor. in .said chamber; an. axially positioned refractory column extending upwardly from said floorl into the` upper sec-tion of said chamber formingianinner` walllfor the gravitating pebble bed; symmetrically arranged pebble outlets in saidi floor spaced.. alternately with. respect to -said ridges and centrallywith respect to the bottom areaof said columnsl so as to provide maximum uniformity of pebbleflow; and gas outlet and pebble inlet means: inthe top of said vessel.

2; The apparatus. of claim 1in which the .gas outlet-means. comprises-an axially disposed stack andsaid pebble inlet means comprises a smaller conduitconcentric with said stack atleast along its-lower end thereby forming an annular gas passagewayaround said conduit vextending, into said-chamber a minor portionof the chamber lengtlrsofasrv to provide a pebble-freey gas collecting 1 space in the s upper portion of said chamber in=communicationwith said` stack; and a pluralityoffuniformly spaced pebble delivery tubes sloping .downwardlyand laterally from the lower endof saidV conduit,` to loci. midway between the upper. end of said refractory column and the inner wall. .of said vessel.A

3: The apparatus of claim l inV which said ridgesare hollowat the lower'end only so as to provide vaf combustion zone thereinin communication with saidheat-exchange chamber through said gasinletsl and. having gas inlets thereto from outside.. said vessel.

4;. The.. apparatus ofv claiml in which said ridgesare. arcuateinhorizontal cross-section.

5. Pebble. heater. apparatus comprising a pair offsu'perposed cylindrical, .refractory-lined, gassolid heat-exchangers in spaced apart relation, each. enclosing aheat-exchangechamber of symmetricalgenerally circular horizontal4 cross-section modied bythe. inward. projection of several verticaluridges uniformly spaced extending upward-ly from theA bottom thereof into the upper section.. of said. chamber so. that Vertical radial planespassing thruvthe center ofgtheridges divideA the chamber-intoa plurality of contiguous vertical columns; a refractory floor in. each o1" saidchambers having several symmetrically arranged. pebble outlets therein disposed alternately around the chamber with respect to said ridgesin .the bottom of said chamber and centrally, with respect tothe bottom, area of said columns so as t provide for maximum uniformity, of pebble flow through the chamber; a verticalV refractory column of minor horizontal cross-sectional'area compared to the correspondingy area of'said''oor extending upwardly into the upper section'of each said chamber from the ii'oor thereof; several pebble inlets in the top ofthe lower chambercorrespondingly positioned to the pebble outletsin the floor above .and in open communication vtherewith providing unobstructed pebble flow between chambers; gasinlets to said chambers inthe lower portion of each of said ridges, the remaining portion of the ridges being imperforate; a. gas outlet in the top of each chamber; a pebble inlet in the top of the upper chamber; a hopper-shaped pebble chamber below saidVV lower chamber communicating with the pebbleoutlets in the floor directly above and converging to a pebble conduit; and elevator means communicating with said pebble conduit andwith the pebble inletiin said upper chamber.

6. The apparatus of claim 5 in which the ridges in said upper chamber. are hollow at the lower end thereof so as to provide aVcombustion-zone in communication with saidV chamber through said gas inletsand having gas inlets thereto from outside said vessel.

7. The apparatus of claim 5 in which thev ridges in said upper chamber are. hollow at the lower end thereof soas to provide a combustion zone in communication with said chamber through said gas inletsand having gas inlets thereto from outside said Vessel.

8.` The apparatusV of claim 5 in which said ridges are arcuate in horizontal cross-section.

9. The'method of efecting heat-exchange between a gravitating compact bed of refractory pebblesand a gas which comprises gravitating said pebbles through a vertical symmetrical generally annular heat-exchange zone surrounding avertical cylindrical zone closed. to the ow of pebbles; withdrawing pebbles from the bottom of said zone through several -pebble passageways spaced symmetrically in said bottom and midway between said cylindrical zone and thel outer boundary of said heat-exchange zone so. as to provide uniform pebble flow in at least theupper three-fourths of said zone; introducing gas at adifferent temperature than the temperature of said pebbles to said zone in the lower portion of the uniform pebble flow section at regularly spaced points around the4 periphery'of saidv heatexchange zone; passing said gas inwardly and upwardly over the entire horizontal cross-section of said zonein direct contact with said pebblesin h'eat-exchangerelation therewith; and recovering said gasfrom theupper' section. of said zone;

10. The. process of. claim 9 in which said gas is-hot combustion gas and said pebblesvare heated thereby and thereafter contacted. in a subja'cent zonewith a cooler gasso as to heat the same;

ll. The :process of claim 10 in which said cooler gas is a hydrocarbon and conversion of said hydrocarbon is effected.

ROBERT. R. GOINS.'

References Cited' in the i-lle of this patent UNITED STATES PATENTS,-

Number Name Date 2,272,108 Bradley Feb. 3, 1942 2,312,006 Thiele Feb. 23, 1943 2,430,669 Crowiey Nov: 1l, 1947 2,432,520 Ferro, Jr. Dec. 16, 1947 2,436,254 Eastwoodet al. Feb. 17, 1948 2,446,388 Ramseyer et al.' Aug. 3, 1948 2,534,625 Robinson Dec. 19, 1950 

9. THE METHOD OF EFFECTING HEAT-EXCHANGE BETWEEN A GRAVITATING COMPACT BED OF REFRACTORY PEBBLES AND A GAS WHICH COMPRISES GRAVITATING SAID PEBBLES THROUGH A VERTICAL SYMMETRICAL GENERALLY ANNULAR HEAT-EXCHANGE ZONE SURROUNDING A VERTICAL CYLINDRICAL ZONE CLOSED TO THE FLOW OF PEBBLES; WITHDRAWING PEBBLES FROM THE BOTTOM OF SAID ZONE THROUGH SEVERAL PEBBLE PASSAGEWAYS SPACED SYMMETRICALLY IN SAID BOTTOM AND MIDWAY BETWEEN SAID CYLINDRICAL ZONE AND THE OUTER BOUNDARY OF SAID HEAT-EXCHANGE ZONE SO AS TO PROVIDE UNIFORM PEBBLE FLOW IN AT LEAST THE UPPER THREE-FOURTHS OF SAID ZONE; INTRODUCING GAS AT A DIFFERENT TEMPERATURE THAN THE TEMPERATURE OF SAID PEBBLES TO SAID ZONE IN THE LOWER PORTION OF THE UNIFORM PEBBLE FLOW SECTION AT REGULARLY SPACED POINTS AROUND THE PERIPHERY OF SAID HEATEXCHANGE ZONE; PASSING SAID GAS INWARDLY AND UPWARDLY OVER THE ENTIRE HORIZONTAL CROSS-SECTION OF SAID ZONE IN DIRECT CONTACT WITH SAID PEBBLES IN HEAT-EXCHANGE RELATION THEREWITH; AND RECOVERING SAID GAS FROM THE UPPER SECTION OF SAID ZONE. 